As organic electro-luminescence devices (hereinafter referred to as “organic EL devices”), those are known in which a hole transport layer, an organic emission layer, an electron transport layer, and a cathode are stacked in this order on a transparent electrode (anode) that is formed on the surface of a transparent substrate. By applying a voltage between the anode and the cathode, light occurs from the organic emission layer. The generated light is transmitted through the transparent electrode and the transparent substrate to be extracted to the exterior.
Organic EL devices are characterized by being self-light-emitting type devices, having emission characteristics with a relatively high efficiency, being capable of emission in various color tones, and so on. Therefore, their application to light-emitting elements in display devices (e.g., flat panel displays) and light sources (e.g., backlights or illuminations for liquid crystal display devices) is considered as promising, and some of that has already matured into practical use. In order to apply organic EL devices to such uses, it is desirable to develop organic EL devices that have good characteristics marked by higher efficiency, longer life, and higher luminance.
There are mainly three factors that govern the efficiency of an organic EL device: efficiency of electrical-optical conversion, driving voltage, and light extraction efficiency.
As for efficiency of electrical-optical conversion, those with an external quantum efficiency over 20% have been reported due to the rise of so-called phosphor materials in recent years. As converted into an internal quantum efficiency, this value would be equivalent to substantially 100%. In other words, there have already been experimental instances where the substantial limit value of efficiency of electrical-optical conversion is reached.
As for driving voltage, devices are becoming available which achieve emission with relatively high luminance at a voltage that is about 10% to 20% greater than the voltage corresponding to an energy gap. In other words, in organic EL devices, there is not much room for efficiency improvement based on reduced driving voltage.
Thus, efficiency improvements in organic EL devices based on improving on the two factors of efficiency of electrical-optical conversion and driving voltage are not highly expectable.
On the other hand, the light extraction efficiency of an organic EL device, which is generally on the order of 20% to 30% although subject to some fluctuation depending on the emission pattern and internal layer structure, leaves room for improvement. The reason for such low light extraction efficiency is that the material(s) composing the sites of light emission and their neighborhood has characteristics such as a high refractive index and light absorption. This causes a problem in that total reflection may occur at interfaces between different refractive indices and light may be absorbed by the material(s), thus hindering effective light propagation to the exterior where emission is to be observed. Consequently, in an organic EL device, non-available light accounts for 70% to 80% of the total emission amount. Thus, there is a very large expectation of improvements in the efficiency of an organic EL device that are based on light extraction efficiency improvements.
Against this background, many attempts at improving the light extraction efficiency have been made up to the present. For example, Patent Document 1 discloses an organic EL device having a diffraction grating for suppressing total reflection at interfaces. Patent Document 2 discloses an organic EL device in which a microlens array is provided on the surface of a transparent substrate. Patent Document 3 discloses an organic EL device having an optical sheet with an optical layer that includes beads dispersed in a binder.